Why New Orleans Levees Failed

"After arriving at the scene a month after the hurricane moved on, researchers say they were surprised at the large number of breaks they found compared with what they had seen in media reports.

"We found dozens of breaches throughout the levee system," says Peter Nicholson, who leads the American Society of Civil Engineers' levee-assessment team.

... In sections where the surge didn't overtop levee segments, water percolated under the sheet pilings through layers of peat, sand, and clay and bubbled up on the other side."